The IL-1 family includes several cytokines whose primary function is to mediate immune and inflammatory responses. The earliest members discovered were IL-1 alpha, IL-1 beta, IL-1 receptor antagonist (IL-1ra), and IL-18 (previously known as IGIF and sometimes IL-1 gamma). Following the discovery of additional proteins with homology to these IL-1 family members, a nomenclature system was adopted in which IL-1 alpha is referred to as IL-1F1, IL-1 beta as IL-1F2, IL-1ra as IL-1F3 and IL-18 as IL-1F4. Seven additional cytokines have been classified as IL-1 family members based on amino acid sequence similarity, identity of gene structure, and predicted or known three-dimensional structure (Sims, J. E. et al., Trends Immunol 22:537, 2001; Dunn, E., et al., Trends Immunol 22:533, 2001; Dunn, E. F., et al., Biochemistry 42:10938, 2003; Schmitz et al. Immunity 23:479-490, 2005).
IL-1 alpha, IL-1 beta and IL-1ra (IL-1F1-3, respectively) bind to receptors that are members of the immunoglobulin superfamily, the 80 kDa type I receptor (IL-1RI) and a 68 kDa type II receptor (IL-1RII), as well as a soluble proteolytic fragment of IL-1RII (sIL-1RII). Binding of IL-1 (alpha or beta) to the type I IL-1 receptor (IL-1R) results in recruitment of the IL-1R homolog, IL-1R accessory protein (IL-1 RAcP or AcP), which does not directly bind the ligands but is required for signal transduction (Sims et al. Trends Immunol 22; 537, 2001); binding of IL-1ra does not. Signaling by IL-18 is very similar, although IL-18 utilizes a different receptor complex (Born, T. L., et al., J Biol Chem 273:29445, 1998). IL-1F5, F6, F8 and F9 make use of the IL-1R-related protein 2 (IL-1Rrp2), with F6, F8 and F9 agonizing this receptor pathway, and IL-1F5 antagonizing it (Debets, R., et al., J Immunol 167:1440, 2001; Towne et al. 2004 J Biol Chem 279(14):13677)
Several members of the IL-1 family (IL-1 alpha, IL-1 beta, IL-18, IL-1F7 and IL-33) are synthesized as precursor molecules that are proteolytically cleaved, by caspase-1 in the case of IL-1 beta and IL-18, and by an unidentified protease or proteases for IL-33, IL-1 alpha and IL-1F7. IL-1ra is activated by signal peptidase cleavage of a short peptide from the n-terminus. However, little is known about what, if any, processing occurs with the remaining family members.